The capacity of this type of gear pump is determined by the tooth depth and tooth width or the like, and the discharge flow rate is determined by the capacity and the rotational speed of the gear (pump rotational speed). If the gear pump is used for instance as an oil pump which supplies lubricating oil to an vehicle engine, the capacity of this oil pump is set such that the quantity of oil required for lubrication can be provided even when the output of the driving engine and the rotational speed of the pump are low. Therefore, if the engine output and the rotational speed of the pump increase, the discharge flow rate will be excessive compared to the required amount, the unadjusted high drive force will be consumed by the oil pump, which may lead to loss of engine output.
A known gear pump which resolves this problem is a variable capacity gear pump where one or both of the drive gear and the driven gear are moved in the axial direction in order to reduce the gear engagement width and the capacity as the pump rotational speed increases (for instance, Japanese Patent Application Laid Open No. 2000-120559 and S57-73880). The gear pump of patent application 2000-120559 has a construction where two side plates on either side in the axial direction of the driven gear are provided, the support shaft of the driven gear is supported by both side plates, a biasing force is applied to the back surface of one side plate, and a pressing force which counteracts the biasing force is applied to the back surface of the other side plate based on the discharge fluid pressure. Thereby the driven gear which is between the two side plates will move in the axial direction to a position which balances the pressing force and the biasing force such that the gear engagement width is changed based on the discharge fluid pressure.
Conventionally, when the pressing force and the biasing force are balanced and the driven gear is stopped, the pressing force and the biasing force will act on the side plates in opposing directions to the driven gear, and therefore the support shaft must support the resistance to the load based on the pressing force and the biasing force. Therefore when the pressing force and the biasing force increase such that the side plates cannot provide resistance to this load, the side plates will move relative to the support shaft and will directly push against the side surface of the driven gear, and thereby a large sliding resistance will occur between the driven gear and the side plate, and the mechanical efficiency of the pump may be reduced.